Clojure Functional Programming & JVM Backend Development · Lesson

Ring Middleware in Depth

Learn how Ring middleware wraps handlers to add cross-cutting behavior like logging, parameter parsing, and session handling in Compojure web apps.

What Is Middleware?

In Ring, a handler is a function that takes a request map and returns a response map. Middleware is a higher-order function that takes a handler and returns a new handler.

This lets you wrap behavior around your core logic without modifying it.

(defn wrap-example [handler]
  (fn [request]
    (handler request)))

The Wrapping Pattern

Every middleware follows the same shape: receive a handler, return a function that takes a request. You can act before or after calling the inner handler.

Before: inspect or modify the request
After: inspect or modify the response

(defn wrap-logging [handler]
  (fn [request]
    (println "Incoming:" (:uri request))
    (let [response (handler request)]
      (println "Status:" (:status response))
      response)))

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